In that paper, Broecker modeled the effects of the expected future increase of CO2 due to humans burning fossil fuels, combined with a natural climate cycle which he estimated based on Greeland ice core records, and tweaked to match the observed temperature record at the time — see figure below:

Broecker’s global temperature prediction [click to enlarge]

This was a very simple model, excluding the effects of the sun, volcanoes, other greenhouse gases, aerosols, and so forth, which Broecker acknowledged:

“In this report only the interaction of the CO2 effect and natural climatic change is considered. As other anthropogenic effects are shown to be significant and as means to quantitatively predict their future influence on global temperatures are developed, they can be included in models such as this.”

As it turns out, Broecker has been fortunate, because the cooling effects of human aerosol emissions have roughly cancelled out the warming effects of human non-CO2 greenhouse gas emissions since 1975, and solar activity has been flat over that period. So the net effect of the factors which he did not take into account has been close to zero. However, Broecker was also smart; the dominant effect on temperature since 1975 has been from CO2, as he expected. It’s better to be lucky than good, but it’s best to be both.

Broecker anticipated the actual increase in CO2 very closely, predicting 373 ppm in 2000 and 403 ppm in 2010 (actual values were 369 and 390 ppm, respectively). Broecker also used an equilibrium climate sensitivity of 3°C for doubled CO2; however, his model’s transient climate sensitivity worked out to be 2.4°C for doubled CO2. Current climate models put equilibrium sensitivty at 1.5 times transient sensitivty, so Broecker effectively underestimated the thermal lag of the climate system, and the equilibrium sensitivity in his calculations was approximately equivalent to 3.6°C for doubled CO2 – a bit higher than today’s best estimates of 2°C transient sensitivity, 3°C equilibrium sensitivity.

We digitized Broecker’s prediction from Figure 1, and compared it to the observed global temperature change since 1975 [see top figure]. We adjusted it slightly to reflect the current atmospheric CO2 concentration (390 ppm) as opposed to his predicted 403 ppm, because we’re interested in the accuracy of Broecker’s temperature predictions, not his CO2 predictions.

As you can see, Broecker’s prediction has matched the net global temperature change quite closely over the past 35 years. His ‘natural cycle’ estimate held his prediction below the actual global temperature increase for most of the period, but as illustrated in Figure 1, he predicted its effects would approach zero after 2000. Not coincidentally, this is when his prediction most closely matches the observed global temperature. Broecker overestimated the amount of global warming by 2010 slightly, by a bit less than 0.2°C. This is probably mainly due to his slight overestimate of climate sensitivity, and potentially due to the increased cooling effects over the past decade.

It’s quite remarkable that a prediction made in 1975 using such a simple model of the climate system could so accurately match the observed global temperature change. It’s a testament to the dominant effect of CO2, and the fact that we have had a solid understanding of the fundamental workings of the Earth’s climate for many decades.

Nevertheless, those who are “skeptical” that humans are driving global warming, including the few climate scientists in this category, often emphasize and exaggerate what we don’t know about how the Earth’s climate functions. In his testimony before US Congress earlier this year, “skeptic” climate scientist John Christy compared the state of climate science research in the 1970s to that today, saying “our ignorance about the climate system is just enormous“.

While it’s true that there remain some features of the climate system which we still don’t fully understand, Broecker’s success illustrates that climate scientists have long had a good grasp on the main drivers of the global climate.

While the “skeptics” enjoy criticizing climate scientists, they rarely put their money where their mouths are in terms of making predictions of their own. We previously examined one of the exceptions – Don Easterbrook, who has been predicting imminent global cooling since approximately 2000. He stood by that prediction in December 2008, presenting his projections of future global temperature change at the American Geophysical Union annual conference. Easterbrook’s model is even simpler than Broecker’s, only taking into account his estimates of past natural climate cycles, assuming they will continue in the future, and effectively presuming that CO2 has no effect on global temperatures (throwing out the centuries-old physics of Tyndall and Arrhenius). Figure 3 compares Broecker’s prediction and two of Easterbrook’s to the observed global temperature.

Broecker’s 1975 prediction was within 0.2°C of the observed global temperature in 2010, while Easterbrook’s, last made in December 2008, were off by 0.3 to 0.5°C. This illustrates the importance of basing future predictions on solid physical footing, and also shows that climate scientists understand the inner workings of the global climate much better than the “skeptics” would have us believe. In fact, in the 1970s, climate scientists understood how the Earth’s climate works better than many “skeptic” scientists do in 2011!

Thanks for the re-post. I had a fun time writing this one – couldn’t resist comparing Broecker’s prediction in 1975 to Easterbrook’s in 2008. Just goes to show the difference between science and pseudo-science.

Sure, let’s tax the AIR, to make the weather colder and let’s let carbon trading markets, politicians and corporations manage the temperature of the planet and then some day the climate will be like it should be; safe, predictable and CONTROLLED. CLIMATE CHANGE WASN’ T SUSTAINABILITY. IT WAS A SPECIFIC CO2 DEATH WARRENT TO ALL.
Thankfully, to us REAL planet lovers, the former believer majority voters you now see, the Climate Change mistakes were just academic exaggerations fueled by political correctness gone mad. It wasn’t a lie or a hoax, it was a tragic exaggeration and exploited by lab coat consultants calling themselves saintly scientists. They had the nerve to call consensus, thousands of “scientist” all with unique and special and personal definitions of CO2’s effects. That wasn’t science, it was a consultant’s w*t-dream….See More

Yes, if we just conserve and sacrifice and do as we are told and tax the air to make the planet colder, then and only then will the poor little planet that produced us will be like it used to be, like it should be, like the inside of an indoor shopping mall? This wasn’t science. It was Omen Worship.
How is fear mongering with exaggeration as motivation for responsible stewardship considered progressive?
How is letting the carbon markets and corporations and politicians rule the temperature of the earth considered progressive?
How is thousands of scientists saying the same thing in thousands of different and unique ways considered science?
Why should we trust a profession such as “scientists” when it was “scientists” who polluted the planet with the pesticides they created and gave us germ warfare, deep sea drilling technology and cruise missiles?
REAL planet lovers don’t want this misery to be true and REAL planet lovers don’t so flippantly believe in CO2 he! and with such childish glee.
At least the neocons didn’t condemn billions of children to a CO2 death knowing full well it was a tragic exaggeration.

Climate Change wasn’t Ozone, or pollution, or energy, or little frightened kids planting trees to SAVE THE PLANET.
NO!
It was a 25 year old CO2 death threat of climate crisis to billions of children and thankfully to us REAL planet lovers and former climate change believers, a tragic exaggeration exploited by the same saintly “scientists” and lab coat consultants that poisoned the planet with their pesticides in the first place.

I dare any of you remaining faded climate blame believers to ACT like it is the crisis you say it is.
Nothing could be worse than a climate crisis besides a comet hit, so since you AND the scientists are not marching and protesting and getting on CNN and warning the world of the biggest emergency ever, we don’t believe YOU or the exaggerated disco science.
How do we have more scientists than protesters anyways?

Marching is for emotional causes. There is no emotion here, just well measured interactions between gases and light. Global warming is about established facts and consequences. Smart people understand facts and consequences. Do you wait for people marching in the street before you pay your electric bill? No, you you just pay it before the power company shuts off the power.

It is not difficult to make a numerical model of a live Earth with an ocean and land biota taking an active part in climate regulation and then try the experiment of adding a terraton of carbon dioxide to the model world as we are doing. I did this in collaboration with the geochemist Lee Kump and we published it in the science journal Nature in 1994 [Failure of climate regulation in a geophysiological model by James E. Lovelock & Lee R. Kump, Nature 369, 732-734, 30 June 1994].

when the CO2 in the air exceeds 500 ppm the global temperature suddenly rises 6 degrees Celsius and becomes stable again despite further increases or decreases of atmospheric CO2. This contrasts with the IPCC models that predict that temperature rises and falls smoothly with increasing or decreasing CO2.

“…why we should take this model seriously when so many of the world’s climate scientists are in agreement on the IPCC predictions. First, although simple, it is a model of the whole Earth system and not merely one based almost entirely on atmospheric physics. Perhaps the most important message from this simple model is its implication that the ocean ecosystems dominate the cooler periods of the Earth’s history and the Land ecosystems the stable hot periods.”